Microfabricated diamond element and method of fabricating microfabricated diamond element

ABSTRACT

A diamond electron emission element is provided with a substrate, and a plurality of quadrangular columns (microscopic projections) composed of diamond and with side faces of flat faces, which are arranged at equal intervals on the substrate. A top end face (horizontal section) is of a quadrangular shape having a length of long sides being a [nm] and a length of short sides being ka [nm], and a thin film of SiO 2  is formed on a side face on the short-edge side. The length a [nm] of long sides and the length ka [nm] of short sides satisfy relational expressions of Formulae (1) and (2) below.
 
 C   1 =2 a √{square root over (1+ k   2 )}  (1)
 
nλ=C 1   (2)
     C 1 : a distance [nm] of a lap in a situation where light generated inside each quadrangular column goes around on a specific circuit while being reflected on the side faces of the quadrangular column,   n: an arbitrary positive integer, and   λ: an emission peak wavelength λ [nm] of the diamond making the quadrangular columns.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microfabricated diamond elements usedin light emitting devices and others, and methods of fabricatingmicrofabricated diamond elements.

2. Related Background Art

The microfabricated diamond elements used in the conventional lightemitting devices was those with PN junctions in diamond columns ofarbitrary shape, for example, as described in New Diamond (Japan Newdiamond Forum) Vol. 17, No. 4 (2001), p10 and subsequent pages (Document2).

Other documents listed below also disclose the techniques related to thepresent invention.

-   [Document 1] Japanese Patent Application Laid-Open No. 2002-075171-   [Document 2] New Diamond (Japan New Diamond Forum) Vol. 17 No.    4 (2001) p10 and subsequent pages-   [Document 3] Extended Abstracts (The 47th Spring Meeting); The Japan    Society of Applied Physics and Related Societies, No. 1 30a-YQ-3    (p377)-   [Document 4] Extended Abstracts (The 46th Spring Meeting); The Japan    Society of Applied Physics and Related Societies, No. 1 30p-M-12    (p415)-   [Document 5] Extended Abstracts (The 62nd Autumn Meeting); The Japan    Society of Applied Physics and Related Societies, No. 3 13a-ZK-5    (p782)

SUMMARY OF THE INVENTION

However, the luminous efficiencies were insufficient with themicrofabricated diamond elements used in the conventional light emittingdevices.

The present invention has been accomplished in view of the above problemand an object of the invention is to provide a microfabricated diamondelement of shape capable of increasing the luminous efficiency.

In order to solve the above problem, a microfabricated diamond elementaccording to the present invention is a microfabricated diamond elementwherein at least one columnar body of a quadrangular cross sectioncomprising diamond is formed on a substrate, and wherein lengths of along side and a short side in the cross section of the columnar bodysatisfy relational expressions represented by Formulae (1) and (2)below;C ₁=2a√{square root over (1+k ²)}  (1)nλ≈C₁  (2)

-   C₁: a distance [nm] of a lap in a situation where light generated    inside the columnar body goes around on a specific circuit while    being reflected on side faces of the columnar body,-   n: an arbitrary positive integer,-   λ: an emission peak wavelength [nm] of the diamond,-   a: the length of the long side [nm], and-   k: a ratio of the length of the short side to the length of the long    side.

In order to solve the above problem, another microfabricated diamondelement according to the present invention is a microfabricated diamondelement wherein at least one columnar body of a substantially regularlyhexagonal cross section comprising diamond is formed on a substrate, andwherein lengths of sides in the cross section of the columnar bodysatisfy relational expressions represented by Formulae (3) and (4)below;C ₂=3√{square root over (3)}b  (3)nλ≈C₂  (4)

-   C₂: a distance [nm] of a lap in a situation where light generated    inside the columnar body goes around on a specific circuit while    being reflected on side faces of the columnar body,-   n: an arbitrary positive integer,-   λ: an emission peak wavelength [nm] of the diamond, and-   b: the length of the sides [nm].

In order to solve the above problem, still another microfabricateddiamond element according to the present invention is a microfabricateddiamond element wherein at least one columnar body of a circular crosssection comprising diamond is formed on a substrate, and wherein when alength of a radius in the cross section of the columnar body is r [nm],and a specific circuit, on which light generated inside the columnarbody goes around while being reflected on a side face of the columnarbody, is represented by a regular polygon in which a distance from acenter to corners thereof is r [nm], the perimeter C₃ [m] of the regularpolygon satisfies relational expressions represented by Formulae (5) and(6) below:3√{square root over (3)}r<C ₃<2πr  (5)nλ≈C₃  (6)

-   n: an arbitrary positive integer, and-   λ: an emission peak wavelength [nm] of the diamond.

Part of the light generated inside the columnar body goes around theinterior of the columnar body while being reflected on the side faces orside face of the columnar body. When the above relational expressionsare satisfied, the distance of one lap around the circuit becomesapproximately equal to an integral multiple of the emission peakwavelength (definition of the “emission peak wavelength” is describedlatter in the explanation of the embodiment) of the diamond. For thisreason, the light of the emission peak wavelength (light travelingnormally to the longitudinal direction of the columnar body and lightincluding a small traveling component in the longitudinal direction ofthe columnar body) resonates inside the columnar body and is extractedto the outside without being attenuated. As a consequence, the luminousefficiency is increased.

The microfabricated diamond element of the present invention ispreferably one wherein each side face of the columnar body is a flatsurface consisting of a diamond crystal face.

The microfabricated diamond element of the present invention is alsopreferably one wherein the diamond crystal face is a (100) face.

When the side faces are fairly flat surfaces consisting of diamondcrystal faces, the light is regularly reflected on the side faces. Forthis reason, the light generated inside the columnar body tends tobecome a standing wave. When the diamond crystal faces are (100) faces,the flat surfaces are formed with best flatness.

The microfabricated diamond element of the present invention ispreferably one wherein a width w₁ of the columnar body is expressed byFormula (7) below;w ₁ =a√{square root over (1+k ²)},  (7)andwherein the width w₁ is not more than 500 nm.

The microfabricated diamond element of the present invention is alsopreferably one wherein a width w₂ of the columnar body is expressed byFormula (8) below;w₂=2b,  (8)andwherein the width w₂ is not more than 500 nm.

The microfabricated diamond element of the present invention is alsopreferably one wherein a diameter of the columnar body is not more than500 nm.

When the width (diameter) of the columnar body is not more than 500 nm,confinement of carriers (electrons or holes) becomes sufficient, toincrease the probability of recombination so as to promote emission oflight. Table 1 presents the relationship between widths (sizes) of thecolumnar body and intensities of cathodeluminescence.

TABLE 1 SIZE 2 μm 1 μm 700 nm 500 nm 300 nm SUBSTRATE INTENSITY 2 3 5 1020 1 Note) intensities are given as relative intensities to that of thesubstrate being 1.

The microfabricated diamond element of the present invention ispreferably one wherein a width w₁ of the columnar body is expressed byFormula (7) below;w ₁ =a√{square root over (1+k ²)},  (7)andwherein a ratio of a height to the width w₁ of the columnar body is notless than 2.

The microfabricated diamond element of the present invention is alsopreferably one wherein a width w₂ of the columnar body is expressed byFormula (8) below;w₂=2b,  (8)andwherein a ratio of a height to the width w₂ of the columnar body is notless than 2.

The microfabricated diamond element of the present invention is alsopreferably one wherein a ratio of a height of the columnar body to adiameter of the columnar body is not less than 2.

When the aspect ratio (the aspect ratio is defined as a ratio of theheight to the width or diameter of the columnar body) is not less than2, carriers (electrons or holes) become unlikely to migrate away to thesubstrate side, so as to increase the probability of recombination tofacilitate emission of light. Table 2 provides the relationship betweenaspect ratios and intensities of cathodeluminescence.

TABLE 2 ASPECT RATIO 0.5 1 1.4 2 3 SUBSTRATE INTENSITY 2 3 5 10 20 1Note) intensities are given as relative intensities to that of thesubstrate being 1.

The microfabricated diamond element of the present invention ispreferably one wherein a ratio of an area of the cross section normal tothe longitudinal direction of the columnar body to an overall exposedarea of the columnar body is not more than 1/10.

When the ratio of the sectional area of the cross section normal to thelongitudinal direction of the columnar body to the overall exposed areaof the columnar body is not more than 1/10, carriers (electrons orholes) become unlikely to migrate away to the substrate side, so as toincrease the probability of recombination to facilitate emission oflight. Table 3 provides the relationship between area ratios andintensities of cathodeluminescence.

TABLE 3 AREA RATIO 1/3 1/6 1/7.6 1/10 1/14 SUBSTRATE INTENSITY 2 3 5 1020 1 Note) intensities are given as relative intensities to that of thesubstrate being 1.

The microfabricated diamond element of the present invention ispreferably one wherein the columnar bodies are arranged at equalintervals.

The above configuration facilitates fabrication of devices to which themicrofabricated diamond element is applied. Especially, when thecolumnar bodies are provided with the periodicity matching thewavelength of the light to be extracted, it becomes feasible to applythe microfabricated diamond element for emitting monochromatic light andto laser devices.

The microfabricated diamond element of the present invention ispreferably one wherein an optically transparent film with a refractiveindex smaller than that of the diamond is formed in part of the sideface of the columnar body.

The light repeatedly reflected on the side faces or side face of thecolumnar body is extracted through the optically transparent film to theoutside.

A production method of a microfabricated diamond element according tothe present invention is a method of fabricating a microfabricateddiamond element, comprising: an etching step of placing a metal incontiguity with a diamond substrate in a reaction chamber and theneffecting reactive ion etching on the diamond substrate in the reactionchamber.

The method enables formation of finely patterned masks that cannot bemade by photolithography.

Another production method of a microfabricated diamond element accordingto the present invention is preferably a method of fabricating amicrofabricated diamond element, comprising: a step of patterning adiamond substrate with microscopic Al dots not more than 500 nm indiameter in an arrayed state; and a step of effecting reactive ionetching on the diamond substrate in a reaction chamber into which aCF₄/O₂ gas is introduced at a flow ratio of CF₄ not more than 3%.

The arrayed columnar bodies are formed by the above production method.They can also be arrayed at intervals matching the wavelength of thelight to be extracted. In this case, by effecting resonance outside, itbecomes feasible to apply the element to devices emitting monochromaticlight and to laser devices. It is advantageous herein to use asingle-crystal substrate or a substrate highly oriented in plane,because orientations of the columnar bodies can be automatically alignedin a post-process.

The production method of the microfabricated diamond element accordingto the present invention is preferably one wherein the etching stepcomprises a step of introducing a CF₄/O₂ gas at a flow ratio of CF₄ notmore than 3% as a reactive gas into the reaction chamber.

If the plasma gas contains only oxygen a number of aciculate portionswill be formed at the tip of one projection; whereas, the addition ofCF₄ 1–3% will result in making a single aciculate portion there for eachtip of the projection.

The production method of the microfabricated diamond element accordingto the present invention is preferably one further comprising: a diamondcrystal face forming step of exposing the diamond substrate withmicroscopic projections formed by the etching step, to a plasma of a gasmainly comprised of hydrogen.

The side faces of the columnar body are reconstructed into diamondcrystal faces.

In order to solve the above problem, another microfabricated diamondelement of the present invention is a microfabricated diamond elementwherein at least one columnar body of a quadrangular cross sectioncomprising diamond and having a maximum diameter of not more than 50 nmis formed on a substrate, and wherein lengths of a long side and a shortside in the cross section of the columnar body satisfy relationalexpressions represented by Formulae (9) and (10) below;nγ≈2a  (9)mγ≈2ka  (10)

-   n: an arbitrary positive integer,-   m: an arbitrary positive integer,-   γ: the de Broglie wavelength [nm] of electrons or holes in the    diamond,-   a: the length of the long side [nm], and-   k: a ratio of the length of the short side to the length of the long    side.

Since the de Broglie wave of electrons (or holes) traveling in thelong-side direction and in the short-side direction forms a standingwave, electrons in the diamond become likely to be excited. The effectbecomes prominent when the columnar body confining electrons (holes) hasa narrow width not more than the maximum diameter of the cross sectionof 50 nm. As a consequence, the luminous efficiency is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a simplified form of diamondelement 1;

FIG. 2 is a schematic diagram showing a simplified form of diamondelement 2;

FIG. 3 is a schematic diagram showing a simplified form of diamondelement 3;

Each of FIGS. 4A–4C shows another embodiment of the microfabricateddiamond element with an isolated particle;

FIGS. 5A and 5B show the shape of end face 12 f of quadrangular column12;

FIG. 6 shows the shape of end face 22 f of hexagonal column 22;

FIGS. 7A–7C show the shape of end face 32 f of circular column 32;

FIG. 8 is a vertical sectional view of transistor 8 to which diamondelement 1 is applied;

FIG. 9 shows the projections formed using circular Al masks;

FIG. 10A shows the quadrangular columns in Example 2, and FIG. 10B is apartly enlarged view thereof;

FIG. 11A shows the end faces of the quadrangular columns in Example 2,and FIG. 11B is a partly enlarged view thereof;

FIG. 12 shows the projections formed using micromasks;

FIG. 13A shows the quadrangular columns in Example 3, and FIG. 13B is apartly enlarged view thereof;

FIG. 14 shows the end faces of the quadrangular columns in Example 3;

FIG. 15 shows the hexagonal columns in Example 4;

FIG. 16 shows the end faces of the hexagonal columns in Example 4;

FIG. 17 is a diagram for explaining the crystal structure in the formwith an isolated particle at the tip;

FIG. 18 shows the quadrangular columns or hexagonal columns in Example5;

FIG. 19 shows the end faces of the quadrangular columns or hexagonalcolumns in Example 5; and

FIG. 20 is a diagram for explaining the crystal structure in the formwith an isolated particle at the tip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings.

First Embodiment

First, the structure of the microfabricated diamond element (diamondelement 1) in the first embodiment will be described. FIG. 1 is aschematic diagram showing a simplified form of diamond element 1. Thediamond element 1 is comprised of a substrate 11, and a plurality ofquadrangular columns 12 (microscopic projections) arranged at equalintervals on the substrate 11, made of diamond, and having side faces offlat faces. The diamond making the quadrangular columns 12 demonstratesluminescence characteristics against excitation means such as electronbeam irradiation, X-ray irradiation, photoexcitation, current injection,heating, or the like and has a broad or sharp overall spectrum ofgenerated light, which has a particularly high luminous intensity at acertain wavelength. In this specification, this wavelength is referredto as an “emission peak wavelength λ [nm]”.

It is also conceivable to design the sectional shape of the columnarbodies (quadrangular columns 12) in agreement with another wavelengthcomponent instead of the emission peak wavelength, and to extract lightof any desired wavelength component, though the luminous efficiencydecreases in this case.

The diamond making the quadrangular columns 12 is preferablyheteroepitaxial diamond, or a highly-oriented diamond film (in whichplane directions are preferably aligned within 5°). There are norestrictions on the material of the substrate 11. In the case of ahighly-oriented diamond film substrate, it is necessary to set the sizeof particles to approximately double the size (width) of theprojections. This prevents electric supply from being hindered byinfluence of grain boundaries.

A thin film 13 of SiO₂ (not shown in FIG. 1) is formed on one side faceof each quadrangular column 12. The quadrangular columns 12 aresurrounded with a substance with a lower refractive index than those ofdiamond and SiO₂. There are no restrictions on the state of thislow-index substance, and the low-index substance may be a gas or anyother solid material. In a preferred example, air pervades the spacearound the quadrangular columns 12. The space around the quadrangularcolumns 12 may be kept in vacuum.

FIG. 5A shows the shape of end face 12 f of quadrangular column 12. Asshown in FIG. 5A, the end face 12 f is of a quadrangular shape havingthe length of long sides being a [nm] and the length of short sidesbeing ka [nm], and SiO₂ film 13 is formed on a side face on theshort-edge side. In this regard, it is noted that the length a [nm] ofthe long sides and the length ka [nm] of the short sides satisfy therelational expressions represented by Formulae (1) and (2) below.C ₁=2a√{square root over (1+k ²)}  (1)nλ≈C₁  (2)

-   C₁: a distance [nm] of a lap in a situation where light generated    inside the quadrangular column 12 goes around on a specific circuit    while being reflected on the side faces of the quadrangular column    12, and-   n: an arbitrary positive integer.

The width w₁ of quadrangular column 12 (defined as a length of adiagonal, as expressed by Formula (7) below) is adjusted so as to be notmore than half of the height of quadrangular column 12 (i.e., so thatthe aspect ratio is not less than 2) and be not more than 500 nm.w ₁ =a√{square root over (1+k ² )}  (7)

The area of end face 12 f is adjusted to be not more than 1/10 of theoverall exposed area of quadrangular column 12.

The operation and effect of diamond element 1 will be described below.Since the lifetime of carriers (electron or holes) is relatively long indiamond having the indirect transition type band structure,electron-hole pairs become easier to recombine to emit light in the casewhere the quadrangular columns 12 are of the vertically long shape withnarrow width to confine carriers (electrons or holes) in the narrowdiamond crystal structure.

Since the quadrangular column 12 is surrounded with the low-indexsubstance, part of light generated inside quadrangular column 12 isrepeatedly reflected on the side faces of quadrangular column 12. FIG.5A shows paths of light arriving at a small angle of incidence on a sideface and traveling while being reflected on the side faces. The distanceof a lap around such traveling paths is equal to the right side of aboveFormula (1), i.e., twice the length of the diagonal of end face 12 f.Therefore, when the length a [nm] of the long sides and the length ka[nm] of the short sides of the end face 12 f are adjusted to satisfy therelational expression of above Formula (1), the light (light travelingnormally to the longitudinal direction of the columnar body or lightincluding a small traveling component in the longitudinal direction ofthe columnar body) becomes a standing wave in the traveling paths. Forthis reason, the light resonates inside the quadrangular column 12, andthe light is extracted to the outside without being attenuated. Thelight generated inside the quadrangular column 12 leaks little by littlefrom the side faces or end face 12 f of quadrangular column 12. In thepresent embodiment, because the SiO₂ film 13 (refractive index: 1.5)with the refractive index smaller than that of diamond (refractiveindex: 2.5) is formed on one side face, the light is likely to emergethrough the SiO₂ film 13 to the outside.

The shape of the end face 12 f may be square (k=1). FIG. 5B shows pathsof light traveling while being reflected on the side faces in the casewhere the end face 12 f is square.

Since the side faces of quadrangular column 12 are flat faces, the lightis regularly reflected on the side faces. For this reason, the lightgenerated inside the quadrangular column 12 is likely to become astanding wave.

For extracting the light including a small traveling component in thelongitudinal direction of the columnar body, from the top surface of thecolumnar body, the top surface of the columnar body is preferably not aflat surface but a projecting shape. In this case, the light can beextracted from the top part, and thus there is no need for forming thematerial with the refractive index smaller than that of diamond on theside face.

The exciting means for luminescence may be cathodeluminescence ofelectron beam irradiation or excitation with X-rays. It may also beexcitation by electric injection into a pn junction or a pin junction.

The diamond element 1 with the quadrangular columns 12 being formed onthe substrate 11 can also be applied to transistors. FIG. 8 is avertical sectional view of transistor 8 to which the diamond element 1is applied. First insulating film 86, gate metal film 84, and secondinsulating film 86 are successively stacked on the substrate 11 so as tofill the space among the quadrangular columns 12. The gate metal film 84is electrically connected to the quadrangular columns 12. An electrodemetal film 82 is formed on the second insulating film 86 so as to beelectrically connected to the quadrangular columns 12. The diamondmaking the substrate 11 and quadrangular columns 12 contains boron andis thus electrically conductive.

There is a Schottky barrier between the gate metal film 84 and thequadrangular columns 12, so that electrons flow from the gate metal film84 with a higher Fermi level into the quadrangular columns 12 to form adepletion layer in the quadrangular columns 12. The thickness of thedepletion layer is increased or decreased by applying a positive ornegative voltage to the gate metal film 84, to control the intensity ofelectric current flowing from the substrate 11 to the electrode metalfilm 82.

The control of the electric current becomes easier as the width ofquadrangular columns 12 decreases. A higher electric current can flowwith increase in the number of quadrangular columns 12.

In the diamond element 1 of the present embodiment, since the side facesof quadrangular columns 12 are flat, good electrical connections areestablished with the gate metal film 84. For this reason, the controlbecomes easier over the thickness of the depletion layer.

Second Embodiment

First, the structure of the microfabricated diamond element (diamondelement 2) in the second embodiment will be described. FIG. 2 is aschematic diagram showing a simplified form of diamond element 2. Thediamond element 2 is comprised of a substrate 21, and a plurality ofhexagonal columns 22 (microscopic projections) arranged at equalintervals on the substrate 21, made of diamond, and having side faces offlat faces. The diamond making the hexagonal columns 22 demonstratesluminescence characteristics against excitation means such as electronbeam irradiation, X-ray irradiation, photoexcitation, current injection,heating, or the like and has a broad or sharp overall spectrum ofgenerated light, which has a particularly high luminous intensity at acertain wavelength (emission peak wavelength λ [nm]).

It is also conceivable to design the sectional shape of the columnarbodies (hexagonal columns 22) in agreement with another wavelengthcomponent instead of the emission peak wavelength, and to extract lightof any desired wavelength component, though the luminous efficiencydecreases in this case.

A thin film 23 of SiO₂ (not shown in FIG. 2) is formed on one side faceof each hexagonal column 22. The hexagonal columns 22 are surroundedwith a substance with a lower refractive index than those of diamond andSiO₂.

FIG. 6 shows the shape of end face 22 f of hexagonal column 22. As shownin FIG. 6, the end face 22 f is of a hexagonal shape having the lengthof each side being b [nm], and SiO₂ film 23 is formed on one side face.The length b [nm] of each side satisfies the relational expressions ofFormulae (3) and (4) below.C ₂3√{square root over (3)}b  (3)nλ≈C₂  (4)

-   C₂: a distance [nm] of a lap in a situation where light generated    inside the hexagonal column 22 goes around on a specific circuit    while being reflected on the side faces of the hexagonal column 22,    and-   n: an arbitrary positive integer.

The width w₂ of hexagonal column 22 (defined as a length of a longestdiagonal, as expressed by Formula (8) below) is adjusted so as to be notmore than half of the height of hexagonal column 22 (i.e., so that theaspect ratio is not less than 2) and be not more than 500 nm.w₂=2b  (8)

The area of end face 22 f is adjusted to be not more than 1/10 of theoverall exposed area of hexagonal column 22.

The operation and effect of diamond element 2 will be described below.Since the hexagonal columns 22 are surrounded with the low-indexsubstance, part of light generated inside each hexagonal column 22 isrepeatedly reflected on the side faces of the hexagonal column 22. FIG.6 shows paths of light arriving at a small angle of incidence on a sideface and traveling while being reflected on the side faces. The distanceof a lap around the traveling paths is equal to the right side of aboveFormula (3). Therefore, when the length b [nm] of each side of end face22 f is adjusted to satisfy the relational expression of above Formula(3), the light (light traveling normally to the longitudinal directionof the columnar body or light including a small traveling component inthe longitudinal direction of the columnar body) becomes a standing wavein the traveling paths. For this reason, the light resonates inside thehexagonal column 22, and the light is extracted to the outside withoutbeing attenuated. The light generated inside the hexagonal column 22leaks little by little from the side faces or end face 22 f of hexagonalcolumn 22. In the present embodiment where the SiO₂ film 23 (refractiveindex: 1.5) with the refractive index smaller than that of diamond(refractive index: 2.5) is formed on one side face, the light is likelyto emerge through the SiO₂ film 23 to the outside. In the other respectsthe present embodiment is able to achieve the operation and effectsimilar to those in Embodiment 1.

Third Embodiment

First, the structure of the microfabricated diamond element (diamondelement 3) in the third embodiment will be described. FIG. 3 is aschematic diagram showing a simplified form of diamond element 3. Thediamond element 3 is comprised of a substrate 31, and a plurality ofcircular columns 32 (microscopic projections) arranged at equalintervals on the substrate 31 and made of diamond. The diamond makingthe circular columns 32 demonstrates luminescence characteristicsagainst excitation means such as electron beam irradiation, X-rayirradiation, photoexcitation, current injection, heating, or the likeand has a broad or sharp overall spectrum of generated light, which hasa particularly high luminous intensity at a certain wavelength (emissionpeak wavelength λ [nm]).

It is also conceivable to design the sectional shape of the columnarbodies (circular columns 32) in agreement with another wavelengthcomponent instead of the emission peak wavelength, and to extract lightof any desired wavelength component, though the luminous efficiencydecreases in this case.

FIG. 7 shows the shape of end face 32 f of circular column 32. As shownin FIG. 7, the end face 32 f is a circle with the radius being r [nm].The radius r [nm] is adjusted so that approximately six times the radiusr becomes equal to an integral multiple of the emission peak wavelengthwithin the scope satisfying the relational expressions of Formulae (5)and (6) below.3√{square root over (3)}r<C ₃<2πr  (5)nλ=C₃≈6r  (6)

-   C₃: a distance [nm] of a lap in a situation where light generated    inside the circular column 32 goes around on a specific circuit    while being reflected on the side face of the circular column 32,    and-   n: an arbitrary positive integer.

The diameter of circular column 32 is adjusted so as to be not more thanhalf of the height of circular column 32 (i.e., so that the aspect ratiois not less than 2) and be not more than 500 nm.

The area of end face 32 f is adjusted to be not more than 1/10 of theoverall exposed area of circular column 32.

The operation and effect of diamond element 3 will be described below.Since each circular column 32 is surrounded with the low-indexsubstance, part of light generated inside the circular column 32 isrepeatedly reflected on the side face of circular column 32. FIG. 7shows paths of light traveling while being reflected on the side face.The distance of a lap around the traveling paths is within the range of3 (3^(1/3))r to 2 πr [nm]. Therefore, when the radius r [nm] of the endface 32 f is adjusted so that approximately six times the radius rbecomes equal to an integral multiple of the emission peak wavelengthwithin the range satisfying the relational expression of Formula (5)above, the light (light traveling normally to the longitudinal directionof the columnar body or light including a small traveling component inthe longitudinal direction of the columnar body) becomes a standing wavein the traveling paths. For this reason, the light resonates inside thecircular column 32, and the light is extracted to the outside withoutbeing attenuated. In the other respects the present embodiment is ableto achieve the operation and effect similar to those in Embodiment 1.

Diamond is the indirect transition type material, which forms excitonswith a very strong bond and brings about exciton luminescence. It alsodemonstrates trace quantity of luminescence of free excitons without anyphonon, and exciton luminescence with n phonons in the TO mode and inthe LO mode. It also exhibits luminescence from droplets consisting of alot of excitons.

Furthermore, if there exists boron or impurities, the excitons will bebound to cause luminescence of bound excitons. Such luminescenceinvolves emission of light at different luminescence peaks, dependingupon states of impurities and others in the material between 230 and 240nm, and it is thus feasible to implement luminescence at variousluminescence wavelengths. If a large amount of boron is includedluminescence can occur at 250 nm. If there exist defects luminescencewill occur in a broad band of about 300–400 nm at room temperature, or abroad band of luminescence called band A will appear near 420 nm. It isalso feasible to implement luminescence at any wavelength in the rangeof 500 nm to 750 nm, such as H3 centers, NV centers, GR centers, etc.due to nitrogen or defects. As described above, it is feasible torealize the luminescence wavelengths ranging from 230 nm to visiblelight. Such luminescence can be combined with geometrically machinedshapes.

FIG. 4 shows another embodiment of the microfabricated diamond element.The microfabricated diamond element of the present invention may be of ashape with an isolated particle of diamond being on the tip of acolumnar body. In this case, the joint area is small between theisolated particle and the tip of the columnar body, so that the effectof confining carriers (electrons or holes) generated in the isolatedparticle becomes prominent.

In the above embodiments, the sectional shape of the columnar body isdesigned so as to induce resonance at the emission wavelength, but itmay also be designed so as to induce resonance at the de Brogliewavelength of electrons or holes in diamond instead thereof. Namely, thesectional shape of the columnar body is designed so as to satisfyFormulae (9) and (10) below. In this case, the width or diameter of thecross section of the columnar body needs to be not more than 50 nm.nγ≈2a  (9)mγ≈2ka  (10)

-   n: an arbitrary positive integer,-   m: an arbitrary positive integer,-   γ: the de Broglie wavelength [nm] of electrons or holes in diamond,-   a: the length of the long sides [nm], and-   k: the ratio of the length of the short sides to the length of the    long side.

Next, an embodiment of the method of fabricating the microfabricatedtransistor element will be described. First, prepared is asingle-crystal diamond substrate, or a substrate of a material exceptfor diamond (Si or the like) with a highly-oriented diamond film or aheteroepitaxial diamond film being formed thereon.

A film of a mask material such as Al, SiO₂, or the like is formed on thesubstrate and is patterned by photolithography technology. Etching iscarried out using the patterned film as a mask, to form diamondprojections with high aspect ratios. Any desired shape (quadrangle,regular hexagon, circle, or the like) and arrangement of projections canbe achieved by the mask patterning.

Thereafter, the projections are processed in a plasma of a gas mainlycontaining hydrogen, to reconstruct diamond crystals, whereby the sidefaces can be formed of diamond crystal faces. In this case, planedirections of diamond affect the shape of deformed projections. Forexample, in the case of a (100) substrate, the projections tend to bequadrangular columns in which (100) faces appear in the side faces andend face. In the case of a (110) substrate, the projections tend to bequadrangular columns or hexagonal columns in which (110) faces or (100)faces appear in the side faces. In the case of a (111) substrate, theprojections tend to be hexagonal columns in which higher-index facesappear in the side faces.

When the projections are made of the highly-oriented diamond film, planedirections of side faces of the rectangular columns are not aligned evenafter the plasma process. In this case, the shape of the projections ismade by only the etching process without the plasma process.

EXAMPLES

The present invention will be described below in further detail withExamples thereof, but it is noted that the present invention is by nomeans intended to be limited to these Examples.

Example 1

First, an Al film was formed on each (100) diamond substrate bysputtering or vapor deposition. The thickness of the Al film isapproximately 500 Å-2 μm, and is determined according to the patterningsize or the height of projections. Since the etching ratio of diamond toAL is 10 or more, the thickness of 0.5 μm was enough for the Al film inthe case of the projections with the height of not more than 5 μm.

Microscopic patterns were formed in the thus formed Al film byphotolithography technology. When the patterns are microscopic, theetching becomes easier with decrease in the thickness of the Al film.The patterns of the end faces of the projections may be quadrangular,but they were circular herein. Using such Al films as masks, the etchingby RIE technology was carried out in a gas containing CF₄ 1–3% inoxygen. If the plasma gas contains only oxygen a number of aciculateportions will be formed at the tip of one projection; whereas, theaddition of CF₄ 1–3% will optimize the number of aciculate portions toone.

The diamond substrates after the etching were subjected to the plasmaprocess under the conditions presented in Table 4.

TABLE 4 PLASMA GAS PRESSURE POWER SUBSTRATE PROCESS NO. AND FLOW RATE(Torr) (W) TEMPERATURE (° C.) TIME 1 H₂:100 sccm 25 1300 750 APPROX. 30MIN 2 H₂:300 sccm 50 1300 750 APPROX. 30 MIN 3 H₂:300 sccm 100 1300 750APPROX. 38 MIN 4 CO₂:3 sccm + 100 1300 750 APPROX. 10 MIN H₂:297 sccm 5H₂:100 sccm 25 1000 900 APPROX. 30 MIN 6 H₂:100 sccm 25 1300 750 APPROX.30 MIN 7 H₂:100 sccm 25 300 750 APPROX. 30 MIN 8 CO₂:3 sccm + 25 1300750 APPROX. 10 MIN H₂:297 sccm 9 H₂:100 sccm 25 1300 750 APPROX. 30 MIN10 H₂:100 sccm 25 1300 750 APPR0X. 30 MIN 11 H₂:100 sccm 25 1300 750APPROX. 30 MIN 12 H₂:100 sccm 25 1300 750 APPROX. 15 MIN 13 H₂:100 sccm25 1300 750 APPROX. 10 MIN

As a result, the circular projections were shaped into quadrangularcolumns in all the samples. Although there were differences in thelength of process time, depending upon the conditions, the shaping wasbasically implemented under any conditions to effect synthesis ofdiamond, provided that the methane concentration was very low and thegas was in a hydrogen excess condition or in an oxygen added condition.With increase in power or with increase in substrate temperature theprocess time became shorter and the shaping became more difficult. Theshaping was infeasible under conditions that diamond was not formedunder substances other than it were formed. Under the conditions of Nos.1, 6, and 9–13 among the conditions presented in Table 4, the resultantside faces of the quadrangular columns became fairly flat. Inparticular, favorable conditions were the pressure condition in therange of 10–30 Torr and the substrate temperature in the range of650–850° C. Among those, the best conditions in terms of controllabilityand fineness of shape were the conditions of the pressure condition of25 Torr and the substrate temperature of 750° C.

Example 2

Circular Al masks arrayed at equal intervals were formed on a (100)diamond substrate by photolithography, and the substrate was subjectedto RIE. As a result, aciculate projections with cylindrical bodies onthe bottom side were formed in an arrayed state. FIG. 9 shows theprojections formed with the use of the circular Al masks. The reason whythe projections look aciculate is that the Al masks were very small andvery deep etching was done in spite of the small masks. When thecircular Al masks are large, cylinder-shaped projections are formed.

The aciculate projections were subjected to the plasma process under thefollowing conditions: H₂ flow rate 100 sccm, pressure 25 Torr, power1300 W, substrate temperature 750° C., and process time 10–15 minutes.As a consequence, the aciculate projections were shaped intoquadrangular columns. This is because the plane direction of thesubstrate was (100). One side of horizontal section of a particularlythin projection was 50 nm. FIG. 10A shows the quadrangular columns inthe present example, and FIG. 10B is a partly enlarged view thereof. Asshown in FIGS. 10A and 10B, diamond was deposited with the spreadingbottom at the root of each quadrangular column.

FIG. 11A shows end faces of the quadrangular columns in the presentexample, and FIG. 11B is a partly enlarged view thereof. As shown inFIG. 11A, many end faces are square, but some are rectangular. This iscaused by variation in size and shape of the circular Al masks. If thepatterning control of circular Al masks is precise about the size andshape, all the quadrangular columns can be made in substantiallyidentical shape and size.

Example 3

A metal as a mask material was placed in contiguity with a (100) diamondsubstrate (within 5 cm) in a reaction chamber, and this diamondsubstrate was subjected to RIE. Broken pieces of the mask materialattached onto the diamond substrate and functioned as micromasks. As aresult, aciculate projections were formed in random arrangement. FIG. 12shows the projections formed by the use of the micromasks. Since thesize of the micromasks formed by the method of the present example wasvery small, the projections were thinner than those in the case ofExample 2 and were thus arranged in a fine array. One side of horizontalsection of a particularly thin projection was not more than 10 nm.

The aciculate projections were subjected to the plasma process under thesame conditions as in Example 2. As a result, the aciculate projectionswere shaped into quadrangular columns. This is because the planedirection of the substrate was (100). One side of horizontal section ofa particularly thin projection after having undergone plasma process was50 nm. FIG. 13A shows the quadrangular columns in the present exampleand FIG. 13B is a partly enlarged view thereof. FIG. 14 shows the endfaces of the quadrangular columns in the present example.

Example 4

A (111) diamond substrate was subjected to the same etching and plasmaprocess (where the process time was about 30 minutes) as in Example 2.As a result, hexagonal columns were formed. FIG. 15 shows the hexagonalcolumns in the present example. FIG. 16 shows the end faces of thehexagonal columns in the present example. As shown in FIG. 16, some endfaces deviated from the regular hexagon, while some other end faces wereof the regular hexagon. The side faces were not low-index faces, butwere fairly flat faces better than those formed by etching.

Also observed were projections which had a small area of a joint partbetween the tip and a main body supporting it, i.e. columnar bodies withisolated particles thereon. FIG. 17 is a diagram for explaining thecrystal structure of the form with the isolated particle at the tip. Asshown in FIG. 17, the isolated particle is formed when the (100) faceappears only at the tip part. By making use of it, the form with theisolated particle at the tip can be fabricated.

Example 5

A (110) diamond substrate was subjected to the same etching and plasmaprocess (where the process time was about 30 minutes) as in Example 2.As a result, quadrangular columns or hexagonal columns were formed. FIG.18 shows the quadrangular columns or hexagonal columns in the presentexample. FIG. 19 shows the end faces of the quadrangular columns orhexagonal columns in the present example. As shown in FIG. 19; some endfaces deviated from the regular hexagon, while some end faces were ofthe regular hexagon. The columns can be made in uniform shape of therectangular hexagon by controlling the size and shape of Al masks so asto be constant. Two of the side faces were (100) faces. The other faceswere (100) faces or other crystal faces that are not always low-indexfaces, but they were fairly flat faces better than the faces formed byetching.

Also observed were projections which had a small area of a joint partbetween the tip and a main body supporting it, i.e. columnar bodies withisolated particles thereon. FIG. 20 is a diagram for explaining thecrystal structure of the form with the isolated particle at the tip. Asshown in FIG. 20, the isolated particle is formed when the (100) faceappears only at the tip part. By making use of it, the form with theisolated particle at the tip can be fabricated.

Example 6

The microfabricated diamond element with the columnar bodies ofcylindrical shape was formed by the etching process similar to that inExample 2. Sample 1 was made using a diamond substrate in which thecontent of boron, nitrogen, etc. was adjusted so as to achieve theemission peak wavelength of 500 nm. Sample 2 was made using a diamondsubstrate in which the content of boron, nitrogen, etc. was adjusted soas to achieve the emission peak wavelength of 400 nm.

Experiment Result 1

The cathode luminescence (a spectral component at the wavelength of 500nm) was observed for the microfabricated diamond element of Sample 1,and the result was that the substrate demonstrated strong luminousintensity and the columnar bodies did stronger luminous intensity.

Experiment Result 2

The cathode luminescence (a spectral component at the wavelength of 400nm) was observed for the microfabricated diamond element of Sample 1,and the result was that neither the substrate nor the columnar bodiesglowed.

Experiment Result 3

The cathode luminescence (a spectral component at the wavelength of 500nm) was observed for the microfabricated diamond element of Sample 2,and the result was that the substrate was dark but the columnar bodiesglowed at strong luminous intensity.

Experiment Result 4

The cathode luminescence (a spectral component at the wavelength of 400nm) was observed for the microfabricated diamond element of Sample 2,and the result was that the substrate glowed at strong luminousintensity and the columnar bodies were dark conversely.

It is understood from the above experiment results that the columnarbodies of the present embodiment efficiently emit the light at thewavelength of 500 nm but attenuate the light at the wavelength of 400nm. This verifies the principle of the present invention that theluminous efficiency is increased by adjusting the shape of the columnarbodies so as to induce resonance of the light at the emission peakwavelength of the material of diamond, in other word by adjusting thediamond so as to match the resonance wavelength of the columnar bodieswith the emission peak wavelength.

It is also seen from the results of Experiment Results 1 and 3 that whenthe resonance wavelength of the columnar bodies agrees with the emissionpeak wavelength, the luminous efficiency in the columnar bodies isincreased by the effect of confining carriers (electrons or holes).

1. A microfabricated diamond element wherein at least one columnar bodyof a quadrangular cross section comprising diamond is formed on asubstrate, and wherein lengths of a long side and a short side in thecross section of the columnar body satisfy relational expressionsrepresented by Formulae (1) and (2) below;C ₁=2a√{square root over (1+k ²)}  (1)nλ≈C₁  (2) C₁: a distance [nm] of a lap in a situation where lightgenerated inside the columnar body goes around on a specific circuitwhile being reflected on side faces of the columnar body, n: anarbitrary positive integer, λ: an emission peak wavelength [nm] of thediamond, a: the length of the long side [nm], and k: a ratio of thelength of the short side to the length of the long side.
 2. Themicrofabricated diamond element according to claim 1, wherein each sideface of the columnar body is a flat surface consisting of a diamondcrystal face.
 3. The microfabricated diamond element according to claim2, wherein the diamond crystal face is a (100) face.
 4. Themicrofabricated diamond element according to claim 1, wherein a width w₁of the columnar body is expressed by Formula (7) below;w ₁ =a√{square root over (1+k ² )},  (7) and wherein the width w₁ is notmore than 500 nm.
 5. The microfabricated diamond element according toclaim 1, wherein a width w₁ of the columnar body is expressed by Formula(7) below;w ₁ =a√{square root over (1+k ² )},  (7) and wherein a ratio of a heightto the width w₁ of the columnar body is not less than
 2. 6. Themicrofabricated diamond element according to claim 1, wherein a ratio ofa sectional area of the cross section normal to the longitudinaldirection of the columnar body to an overall exposed area of thecolumnar body is not more than 1/10.
 7. The microfabricated diamondelement according to claim 1, wherein the columnar bodies are arrangedat equal intervals.
 8. The microfabricated diamond element according toclaim 1, wherein an optically transparent film with a refractive indexsmaller than that of the diamond is formed in part of the side face ofthe columnar body.
 9. A microfabricated diamond element wherein at leastone columnar body of a substantially regularly hexagonal cross sectioncomprising diamond is formed on a substrate, and wherein lengths ofsides in the cross section of the columnar body satisfy relationalexpressions represented by Formulae (3) and (4) below;C ₂=3√{square root over (3)}b  (3)nλ≈C₂  (4) C₂: a distance [nm] of a lap in a situation where lightgenerated inside the columnar body goes around on a specific circuitwhile being reflected on side faces of the columnar body, n: anarbitrary positive integer, λ: an emission peak wavelength [nm] of thediamond, and b: the length of the sides [nm].
 10. The microfabricateddiamond element according to claim 9, wherein a width w₂ of the columnarbody is expressed by Formula (8) below;w₂=2b,  (8) and wherein the width w₂ is not more than 500 nm.
 11. Themicrofabricated diamond element according to claim 9, wherein a width w₂of the columnar body is expressed by Formula (8) below;w₂=2b,  (8) and wherein a ratio of a height to the width w₂ of thecolumnar body is not less than
 2. 12. A microfabricated diamond elementwherein at least one columnar body of a circular cross sectioncomprising diamond is formed on a substrate, and wherein when a lengthof a radius in the cross section of the columnar body is r [nm], and aspecific circuit, on which light generated inside the columnar body goesaround while being reflected on a side face of the columnar body, isrepresented by a regular polygon in which a distance from a center tocorners thereof is r [nm], the perimeter C₃ [m] of the regular polygonsatisfies relational expressions represented by Formulae (5) and (6)below:3√{square root over (3)}r<C ₃<2πr  (5)nλ≈C₃  (6) n: an arbitrary positive integer, and λ: an emission peakwavelength [nm] of the diamond.
 13. The microfabricated diamond elementaccording to claim 12, wherein a diameter of the columnar body is notmore than 500 nm.
 14. The microfabricated diamond element according toclaim 12, wherein a ratio of a height of the columnar body to a diameterof the columnar body is not less than
 2. 15. A microfabricated diamondelement wherein at least one columnar body of a quadrangular crosssection comprising diamond and having a maximum diameter of not morethan 50 nm is formed on a substrate, and wherein lengths of a long sideand a short side in the cross section of the columnar body satisfyrelational expressions represented by Formulae (9) and (10) below;nγ≈2a  (9)mγ≈2ka  (10) n: an arbitrary positive integer, m: an arbitrary positiveinteger, γ: the de Broglie wavelength [nm] of electrons or holes in thediamond, a: the length of the long side [nm], and k: a ratio of thelength of the short side to the length of the long side.